In many processing industries, “clarification” and/or “decolourisation” are often key processes for purifying a particular product. These terms refer to processes which are capable of removing a range of impurities which vary depending on the process stream being refined.
For example, (but not wishing to be limited by this example), clarification and decolourisation are key unit operations in the sugar processing industry. Indeed, in the sugar processing industry three main processes have been developed to simultaneously clarify and decolorize streams containing sugar. These processes are referred to as (i) carbonatation, (ii) phosphatation and (iii) sulphitation.
Essentially all these processes are similar in that they involve lime reacting with an acid to form respectively calcium carbonate, calcium phosphate and calcium sulphate. Each of these methods has their particular advantages and disadvantages.
Carbonatation is a particularly good process for relatively highly turbid and/or highly coloured streams. It is relatively expensive in terms of set up costs (capital expenditure), especially in view of the need to provide high pressure filtration systems. However, it is relatively inexpensive in terms of operating costs, for example, the acid used is carbonic acid which is readily available from cheap or even free sources. From a sustainability point of view, carbonatation captures CO2, but it also leads to the production of a lot of waste, mainly in the form of calcium carbonate cake which is “contaminated” with the impurities and colour bodies removed from the stream being treated.
Phosphatation, in contrast, is a process which is relatively inexpensive to set up, and which produces significantly less waste than the carbonatation process. One drawback is that the phosphatation process uses phosphoric acid which is relatively expensive and which is becoming more and more scarce and difficult to source. Another drawback is that the phosphatation process requires the addition of relatively expensive colour precipitating chemicals to achieve similar purification performance to the carbonatation process. Additionally, if the phosphatation process is not closely controlled, waste water from this process can be relatively high in volume and also partially contaminated with phosphates.
Sulphitation is a process that is similar to carbonatation, but it has the added disadvantages that it can leave residual SO2 and/or sulphur in the product, it can cause increased scaling in equipment, and the presence of sulphur in exhaust gases can be an environmental nuisance.
Carbonatation and sulphitation have been used in the sugar processing industry for over 100 years. Phosphatation has been used since around 1918, with the current widely used form of phosphatation (as disclosed in U.S. Pat. No. 3,698,951) used since 1971.
The purpose of all of these discussed processes is to remove solid impurities from the stream to be treated (such as sugar). Therefore, unsurprisingly, they all produce solid waste product streams that need to be disposed of. Importantly, the discussed processes of carbonatation, phosphatation and sulphitation are traditionally performed as separate and discrete processes and thus the waste product streams that they produce differ from each other in their by-product compositions. In the case of carbonatation, the by-product is a calcium carbonate cake. In the case of phosphatation, the by-product is calcium phosphate “scum” or “mud”. In the case of sulphitation, the by-product is calcium sulphate “mud”.